Serial communication technique

ABSTRACT

Information can be communicated back from a receiver to a transmitter over a serial link designed for communication to be essentially only in one direction without requiring a separate feedback path with an extra wire. This is achieved by changing at least one characteristic of the receiver input. For example, the receiver input impedance, the receiver input voltage, or both, may be altered. The change in a characteristic of the receiver input can be detected by an appropriately designed transmitter, and such detection can be used to trigger a change in the behavior of the transmitter. Advantageously, only simple alterations are required in conventional serial link circuits to provide a low bandwidth reverse communication path over the same wires. Such a change in a receiver input characteristic can also be employed to inform a transmitter of the existence of a receiver that is on a card that is hot-plugged into system.

TECHNICAL FIELD

This invention relates to the providing of reverse communication over aserial link designed substantially only for communication in onedirection.

BACKGROUND OF THE INVENTION

Many systems employ serial communications. In certain applications theserial link is designed for communication to be essentially only in onedirection. However, at startup, or in the event of an error condition,it may be necessary to communicate some information back from thereceiver to the transmitter. Also, if a card containing a serialreceiver is hot-plugged into a system, the receiver needs to inform thetransmitter of its existence. Conventionally, to implement this type ofreverse communication requires a separate feedback path with an extrawire. If such a serial link with reverse communication is implementedfor communication between integrated circuits, disadvantageously,additional pins are required on each of the communicating integratedcircuits.

SUMMARY OF THE INVENTION

In accordance with the principles of the invention, information can becommunicated back from a receiver to a transmitter over a serial linkdesigned for communication to be essentially only in one directionwithout requiring a separate feedback path with an extra wire. This isachieved by changing at least one characteristic of the receiver input.For example, the receiver input impedance, the receiver input voltage,or both, may be altered. The change in a characteristic of the receiverinput can be detected by an appropriately designed transmitter, and suchdetection can be used to trigger a change in the behavior of thetransmitter. Advantageously, only simple alterations are required inconventional serial link circuits to provide a low bandwidth reversecommunication path over the same wires. Such a change in a receiverinput characteristic can also be employed to inform a transmitter of theexistence of a receiver that is on a card that is hot-plugged intosystem.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing:

FIG. 1 shows an exemplary serial link arranged in accordance with theprinciples of the invention.

DETAILED DESCRIPTION

FIG. 1 shows exemplary serial link 100 arranged in accordance with theprinciples of the invention. The serial link of FIG. 1 includestransmitter line driver 101, serial line 105, receiver unit 107, resetdetermining unit 109, reverse signaling unit 111, reverse signaldetector 113, and data transmit unit 115. Serial link 100 is designedfor essentially unidirectional communication, specifically, only fromtransmitting line driver 101 to receiver unit 107. However,occasionally, a signal may be transmitted back from the receiver side ofserial link 100 to the transmitter side of serial link 100 in accordancewith the principles of the invention.

Operation of the serial link of FIG. 1 is as follows. Data transmit unit115 supplies data to transmitter line driver 101 for transmission to thereceiver side. Transmitter line driver 101 converts the bits receivedfrom data transmit unit 115 to line voltages. Transmitter line driver101 has output impedance 103, e.g., 50 ohms, to provide protection oftransmitter line driver 101 and to provide impedance matching for thetermination of serial line 105.

Data transmitted via serial line 105 is received at receiver unit 107,where, under most circumstances the data is extracted and the clock isrecovered. Transmitter line driver 101, output impedance 103, serialline 105, receiver unit 107 and data transmit unit 115 are known in theart.

In accordance with the principles of the invention, when a predefinedcondition, such as a) an error occurring in transmission, e.g., loss ofsynchronization, or b) when the receiver is hot-plugged into a system,this is detected by, or signaled to, reset determining unit 109. Inresponse, reset determining unit 109 signals reverse signaling unit 111to send a signal back to the transmitter by changing at least onecharacteristic of the receiver input, in accordance with the invention.For example, the receiver input impedance, the receiver input voltage,or both, may be altered. Reset determining unit 109 may also signalreceiver unit 107 to change its operating mode in anticipation of datatransmit unit 115 changing its operating mode as its response toreceiving the reverse signal.

In the exemplary embodiment of FIG. 1, when signaled to do so by resetdetermining unit 109, reverse signaling unit 111 shorts serial line 105to ground, thus changing both the input impedance and the input voltage.This change is detected by reverse signal detector 113, and anindication of such detection is supplied to data transmit unit 115. Forexample, the indication may be supplied as a reset signal to datatransmit unit 115, which causes data transmit unit 115 to reinitializeitself, and, optionally, to execute a startup sequence to initializeserial link 100.

Reverse signal detector 113 may be, for example, a Schmitt trigger. Inanother embodiment of the invention, a comparator may be employed.Alternatively, reverse signal detector 113 may also be arranged todetect reflections back from transmission line 105, so as to detect achange in impedance caused by reverse signaling unit 111.

The techniques of the invention may be employed for single ended or fordifferential serial links.

The foregoing merely illustrates the principles of the inventions. Itwill thus be appreciated that those skilled in the art will be able todevise various arrangements which, although not explicitly described orshown herein, embody the principles of the invention and are includedwithin its spirit and scope.

What is claimed is:
 1. Apparatus for use in serial communication,comprising,a transmitter line driver having a finite output impedanceand adapted for coupling to a serial communications line; and a detectorfor detecting a change of at least one characteristic at a remote end ofsaid communications line, wherein said transmitter line driver is partof a transmitter unit and further including reset circuitry forresetting said transmitter unit in response to said detector detectingsaid change, said reset circuitry being operable independent of anyclock of which an indication is transmitted by said transmitter linedriver.
 2. The invention as defined in claim 1 wherein said at least onecharacteristic is a voltage of said communications line.
 3. Theinvention as defined in claim 1 wherein said at least one characteristicis the impedance at the remote end of said communications line.
 4. Theinvention as defined in claim 1 wherein at least one characteristicincludes both the impedance at the remote end of said communicationsline and a voltage of said communications line.
 5. The invention asdefined in claim 1 wherein said detector detects said change by sensingchanges in the voltage on said communications line.
 6. Apparatus for usein serial communication, comprising,a transmitter line driver having afinite output impedance and adapted for coupling to a serialcommunications line; and a detector for detecting a change of at leastone characteristic at a remote end of said communications line, whereinsaid detector detects said change by sensing reflections on saidcommunications line.